1. Field of the Invention
The subject invention pertains to the field of radar clutter cancellation and more particularly to the adaptive cancellation of clutter with non-zero mean frequencies.
2. Description of the Prior Art
Ground based air surveillance radar systems require the elimination of unwanted echo returns, commonly referred to as clutter, to reliably detect and track airborne targets of interest. Many sources of clutter exist, however two, land and weather, are dominant and present serious impediments to tracking airborne targets. Land clutter is created by radar reflections which, with respect to the radar platform, are stationary or move at velocities that are slow relative to the velocity of the airborne targets. These reflections, therefore, exhibit a doppler shifted frequency spectrum clustered about zero frequency. Since the mean frequency of the land clutter spectrum remains at zero, all radar clutter returns may be suppressed by a fixed MTI canceller.
Weather clutter is created by reflections from wind driven clouds and precipitation, causing the main doppler frequency of the weather clutter spectrum to be non-zero. This shift of the mean doppler frequency is a function of the wind velocity, which may vary with the range-azimuth position of a target of interest. Since this doppler frequency is not known a priori, a single fixed MTI canceller cannot be designed to suppress the weather clutter. Consequently, an adaptive technique must be employed to eliminate this form of clutter.
The first adaptive MTI processors were developed for the elimination of ground clutter in airborne radar systems, which due to the velocity of the aircraft exhibit non-zero mean doppler frequencies. A system of this type, TACCAR, is described by Skolnik in "Radar Handbook", McGraw Hill, New York, 1970. Adaptive clutter rejection systems for shipborne and land based MTI systems have subsequently been utilized for rejecting chaff, sea, and weather clutter. These systems which operate at i.f. frequencies, exhibit blind speeds, utilize analog circuitry, are either extremely complicated or can be used only in a single delay line configuration, thus limiting the clutter cancellation capability, provide cancellation only on a pulse-to-pulse basis, and do not perform well with pulse compression systems.